Social network characteristics, or the patterning of one's social ties, are an important indicator of social support for persons with serious mental illness. Yet relatively few studies have been done on the particular network characteristics of clients receiving case management services in community settings. An understanding of the range of potential and actual social network contacts for persons with serious mental illness is important for targeting interventions to enhance their community supports. In this study, 125 clients of two urban community treatment teams were surveyed by case managers to determine their overall social network size, personal or support network size, and participation in any of 11 network clusters. Results of the study are presented with implications for the social functioning of persons with serious mental illness and suggestions for case manager interventions toward network expansion.